I have a '87 560SL and getting some clattering when I start it from cold. It only lasts about 1 second, but have heard the horror stories of people ignoring the signs pointing to changing the timing chain. My mechanic tells me to do a full job (rails and all with a double chain) the cost would be about $2000. Just to replace the timing chain with a single and NOT replace the rails, etc. would be $1200. Are these prices in line or not?